Silicon steel with improved magnetic anisotropy and method of making the same



United States Patent SILICON STEEL WITH IMPROVED MAGNETIC ANISOTROPY AND METHOD OF MAKING THE SAME Elmore J. Fitz, Lanesboro, Mass., assignor to General Electric Company, a corporation of New York No Drawing. Application December 31, 1956 Serial No. 631,447

9 Claims. (Cl. 148110) The present invention relates to magnetic silicon steel for electrical uses, such as in transformers, motors and other electromagnetic apparatus, and more particularly to such electrical steel having improved grain orienta- Patented Nov. 17, 1959 "ice verse effect on the development of a high degree of magnetic anisotropy unless controlled within narrow limits.

' Sulfur has been recognized as a magnetically undesirable tions. Since .the magnetic properties of single crystals are dependent upon the crystallographic direction, a piece ponent crystals will also have direction-dependent magnetic properties. It is known in the art that it is desirable to provide electrical sheet steel with a high degree of texture, i.e., preferred grain orientation, in order to obtain improved magnetic properties in the steel sheet and thereof strip having a non-random arrangement of its com- 1 V by increase the efiiciency of electrical apparatus in which it is used.

Various metallurgical processes have been carried out in the past for obtaining orientation of the crystals comand carefully controlled processing of thick oriented steel.

It is an object of the invention to provide an improved method of producing electrical steel having a high degree of magnetic anisotropy.

It is another object of the invention to provide an improved process for developing magnetic anisotropy in the steel strip which avoids the necessity for careful selection of the starting material with respect to its constituents and does not require precise control of the processing steps.

It is a further object of the invention to provide thin gauge electrical steel having a high degree of orientation and in improved and simplified method of making the same.

In attempting to develop and improve magnetic aniso-' tropy in finished steel strip, the prior art has approached the problem primarily from the aspect of physical processing and heat treatments of the silicon steel, including I cold rolling and annealing in multi-stage processes.

Since it has been known that various elements such as carbon and metallic oxides have an adverse effect on the development of magnetic anisotropy prior methods have sought to remove such retarding elements or compounds.

It is known thatnitrogen or, nitrides may have an admaterial and efforts are generally made to keep it to a minimum.

It has, however, been unexpectedly found in accordance with the invention that the addition of sulfur, such as by means of metallic sulfides, to the molten iron prior to the usual processing steps has a markedly favorable influence in promoting secondary grain growth and consequent high orientation of the crystals in the finally processed steel strip. By secondary grain growth or secondary recrystallization is meant the process whereby in the final texture-producing annealing treatment, strain-free crystal grains grow in size by absorbing each other. Such secondary grain growth follows primary recrystallization, which is defined as a process whereby the distorted grain structure of a cold-worked metal is replaced by a new strain-free grain structure by annealing above a specific minimum temperature. It is such secondary recrystallization that produces the highly preferred orientation sought in high quality magnetic strip, and the orientation thus obtained is completely different from that obtained merely after primary recrystallization. In general, at a given temperature primary recrystallization will occur in much less time than is required for secondary recrystallization.

' It has been further found in accordance with the invention that by adding sulfur to silicon iron melts the steel thus formed can be made to respond to processing of the prior art type and the crystalline behavior will not be different in thinner material than it is in thicker material, contrary to the teaching of the prior art.

As a further consequence of the invention, it is possible to ease the melting, rolling, and heat treating restrictionsimposed on producers of magnetic steel strip in order to obtain good anisotropy in the final product.

-In conventional steel processing methods, the initially formed steel strip containing about 3% silicon is hot rolled and then subjected to a series of cold rolling and intervening annealing treatments to reduce the gauge of the strip and to remove carbon, sulfur, oxygen, and other elements, and also to control the amount of nitrogen at a suitable level. If sulfur in certain critical amounts is added, in accordance with the invention, to the iron melt prior to its formation into a sheet or strip and the silicon steel is thereafter subjected to the usual rolling, annealing and purifying treatments, the grain orientation of the metal is substantially improved over that of silicon steel to which sulfur has not been similarly added. It appears, also, that the incorporation of sulfur in the amounts disclosed herein does not adversely affect the hot or cold-working of the iron-silicon-sulfur alloys of the present invention. It was found further that the total sulfur content could be finally reduced to the necessary low levels by the usual refining procedures, even in view of the addition of sulfur as disclosed herein.

In the course of experimental work leading to the present discovery, it was noted that improved orientation was produced in silicon steel ingots made with relatively large additions of molybdenum sulfide, e.g., .025- .50% by weight. In a series of tests made to further investigate this discovery and to determine the influential agent in this compound, a number of ingots of silicon steel were made to which sulfur was added by means of different sulfides, viz., molybdenum sulfide, iron sulfide,

'The ingots were made from startingiron, material (raw iron) as received from the supplier having a normal sulfur content of about .025-.035% by weight, and were prepared by melting down the iron, adding about 3% f sili on, and then n o uc ng the s l de c mp u d- The m lts we e p u d i ed a e y aft r the u fid addition. The poured ingots were hot rolled to a band of 100 mils thickness, then cold rolled to 14 ,mil strip in the usual two-stage process including intervening heat treatments (anneals), and finally subjected to box annealing in dry hydrogen at 1100 C. The finished strips were then given the integrated disc test, described below, to determine the degree of anisotropy. Chemical analysis was conducted for total sulfur, as well as other constituents.

The integrated disc test I.D. T.) involved the use .of a spinning disc tester which is an apparatus for the rapid measurement of magnetic anisotropy in sheet or strip material. The tester comprises a motor having a shaft to which is attached a chamber which holds the specimen in the form of a thin disc. The disc is spun ata uniform speed about an axis normal to the surface of the .disc and passing through its center. The device also includes a magnetic .circuit which provides a constant field parallel to the plane of the disc, a set of search coils arranged about the disc in such manner that'any change in the magnetic flux passing through the disc will produce a voltage in the coils, and an oscillograph or other device for detecting any voltage developed across the search coils due to flux variations through the disc. The instrument is calibrated by measuring the voltage developed by specimens whose magnetic anisotropy had been determined by known methods. Since the degree of magnetic anisotropy is a function of the amount of texture, i.e., crystal orientation in the sheet or strip, it is possible to calibrate the spinning disc tester in terms of crystal orientation. The data presented in the table below were obtained from an instrument so calibrated and are in terms of a percentage of perfection that was obtained. Perfection was considered to be a disc which had been prepared from a single crystal of the material under examination. In order to obtain an overall picture of average sheet or strip orientation, the disc size was so chosen that several individual crystals were contained in the specimen.

Results of the above tests are shown in Table I as follows:

Table l Total Sulfur Percent Sulfur A dded, Aniso- Recov- Ingot No Percent Sulfide tropy ered,

by (I.D.T.) Percent weight by weight 0. 003 M052 45 0. 028 0. 006 M032 56 O 056 0.010 IWOSz 59 O. 040 0. 100 M08; 62 0. 078 0. 006 Yes 43 0. 037 0.010 FeS 0. 038 0v 040 Fees 64 0.068 0. 100 FeS 54 ,0. 107 0. 006 A gzS 56 [Jv 034 0.010 A gzS 61 O. 038 O. 040 A gas 82 0. 066 0. 006 GuS 64 0. 039 0v 010 GuS 62 0. 039 0. 040 C118 66 0. 066 O. 100 Gus 66 0.115

As is evident from the above data, the anistoropy in magnetic steel strip increases with increasing additions of sulfur. This data, as well as those from other tests, indicate that the metal ion in the added sulfide compound has little or no influenceon the orientation obtained and that it is the sulfide radical primarily that influences the grain orientation. From Table I is apparent that the optimum amount of sulfur addition is about .04% by weight, although variations from this proportion also produce substantial improvement in orientation.

In general, it has been found that a range of about .005% to .1% by weight of added sulfur gives satisfactory results in accordance with the invention. Below that range of added sulfur the improvement in texture is negligible, whereas additions of sulfur above that range bring about little or no additional improvement.

' Considering that steel produced from similar melts which initially contained as much as .035 sulfur, but to which no sulfide additions were made as described herein, had orientation values .of not more than 50%, it is evident that the sulfur additions as provided herein furnished an effective means for forcing improved orientation in magnetic steel.

It had been observed that the sulfur content of the raw iron in its initial state did not afford an accurate indication of the degree of grain orientation in the finally processed steel strip. It was found, for example, that difierentsamples .of raw iron with practically the same sulfur content, .e.g., even greater than .03%, produced 'tmder the same processing conditions final strips having widely differing anisotropic properties, some of the processed samples having excellent grain orientation characteristic of .the best quality electrical steels, while other samples had extremely poor grain orientation.

It has been further found that if raw iron material with as much as .035 initial sulfur content fails to yield good orientation in the finished strip, the addition of as little as 0.06% sulfur in accordance with the invention will provide substantial improvement in orientation.

The reasons underlying the above effects are not fully understood- A poss b expl n o m y ie i t p rticular state or distribution of the sulfur in the iron melt, so that as between two samples of raw iron having the same initial sulfur content, the sample giving better orientation m y have t su fu in t p op o m t p odu e the desired results. It appears that in any case the addition of sulfur to the starting iron melt in the ran of proportions ,as described herein will produce substantial improvement in the final orientation of the steep strip, regardless .of the original content of sulfur in the raw iron. The present process thus makes it possible to obtain magnetic steel strip of good .quality from raw iron of so-called poor grades, and the careful selection of the starting iron 'or the close control of processing condi tions to ensure good .quality strip as heretofore required in 'prior procedures can largely be dispensed with.

' While the present process is suitable for use with raw iron having an appreciable initial sulfur content, it is preferred to start with iron as free as possible from sulfur, since the subsequent purification of the strip after development of the desired texture is thereby facilitated.

As has already been indicated, it has been observed in the past that good orientation in steel strip is accompanied by the development of secondary recrystallization in the strip, i.e., a growth of crystals in the steel strip during the processing anneals to larger sizes, of a diameter, say, at least A inch, and preferably inch diameter or larger. With this in mind, the samples of strip produced by the ingots shown in Table I were examined for crystal size. This examination showed that samples 393, 396, 397, 399, 403 and 404 had crystals generally larger than those samples having sulfur additions of lesser amounts, and demonstrated by this aspect the improved quality of steel strip obtainable by the present invention. In particular, sample 400 which produced the extremely high anisotropy value of 82% was characterized by even larger sized crystals, representative of the type found i the highest quality magnetic steel strip.

As is well known, thin gauge magnetic strip is desirable for use in electrical induction apparatus in view of the lower eddy current losses which can be thereby obtained. As has been previously mentioned, difficulties have been experienced in the industry in attempts to obtain very thin gauges of silicon steel (e.g., less than 7 mils) having the desired degrees of preferred orientation,

such as characterized the thicker gauges, e.g., 12 mils or higher. In one known method, thin gauge steels have been provided with suitable orientation by employing a series of carefully controlled processing steps to which conventionally oriented strip of substantial thickness, i.e., over 7 mils, is subjected.

In accordance with the present invention, it has been found possible to obtain considerably increased grain orientation in strips rolled directly and without intermediate development of preferred orientation from 100 mil thick bands composed of an iron-silicon-sulfur alloy in accordance with the invention. For example, whereas a sample of conventional high quality silicon steel strip formed to 6 mils by the same direct rolling process produced an orientation of 13%, samples formed from the present iron-silicon-sulfur alloy had in general substantially improved orientation characteristics, and, in fact, values of up to 63% of complete orientation were obtained. Further, in 3 mil strip where the conventional high quality strip had only 8% magnetic anisotropy, values as high as 60% were produced by a sample of the present silicon-iron-sulfur alloy. In these tests it appeared that a low oxygen content was also desirable to obtain high anisotropy. Where the samples had similar oxygen contents (e.g., 0.012%), those that had higher sulfide contents appeared to produce better orientation.

The discovery that a high percentage of preferred crystal orientation can be achieved by means of secondary grain growth in thin gauge magnetic strip merely by adding sulfur in suitable amounts prior to processing the strip makes it possible to obtain oriented thin gauge strip With the usual processes employed for thicker strip and without first preparing oriented thick gauge strip, as found necessary in the prior procedures. A typical process which may be used, and which is given by way of example only, is as follows:

Raw iron scrap containing the usual proportion of impurities such as phosphorus, sulfur, chromium, nickel, aluminum, and copper, as Well as manganese, is melted in a crucible and about 1 to 4% by weight of silicon (preferably about 3%%) is added thereto. To this melt is added a metallic sulfide, such as manganese sulfide, in sufficient amount to provide from .005 to 0.1% sulfur by weight of the melt. With temperature of the melt adjusted to about 1600 C., the mixture is poured into an ingot mold. After solidification, the ingot is hot rolled to a strip 100 mils thick, and then cold rolled to an intermediate gauge, e.g., 30 mils. The strip is then heat treated in an open anneal, cooled and cold rolled to the desired final gauge. The strip is then decarburized by heat treatment in wet air at 800 C. and thereafter subjected to anneal at about 1100 C. for a sufiicient period to grow secondary crystals of optimum size and proper orientation and to further purify the strip by the removal of residues of carbon, sulfur, oxygen and other impurities.

To obtain the benefits of the present invention it is not essential that all of the above alternating annealing and rolling steps be used, the invention in its broad aspects contemplating merely the forming from the ironsilicon-sulfur melt a steel strip which may be rolled directly to the desired gauge and then annealed to refine it and develop the proper grain orientation therein.

The expressions sheet material and sheet as used in the appended claims are intended to include such forms as sheets, strips, tapes and other laminar shapes.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of making silicon steel sheet material comprising forming a melt containing iron, about 14% silicon and sulfur wherein about 0.005 to 0.1% by Weight of sulfur has been added to the melt, producing a metal sheet from said melt, and annealing said metal sheet to refine the same and to develop grain-orientation therein.

2. Silicon steel sheet material less than 7 mils thick as made by the method defined in claim 1.

3. The method of making silicon steel sheet material comprising forming a melt containing iron and adding thereto about 1 to 4% silicon and about 0.005 to 0.1% sulfur, producing a metal sheet from said melt, and annealing said metal sheet to refine the same and to develop grain-orientation therein.

4. The method of making silicon steel sheet material comprising forming a melt containing iron and about 1 to 4% silicon, adding thereto a sufficient amount of a metallic sulfide to provide about 0.005 to 0.1% sulfur, producing a metal sheet from said melt, and annealing said metal sheet to refine the same and to develop grainorientation therein.

5. The method of making silicon steel sheet material comprising forming a melt containing iron and about 1 to 4% silicon, adding thereto a sufficient amount of a metallic sulfide to provide about 0.04% sulfur, producing a metal sheet from said melt, and annealing said metal sheet to refine the same and to develop grainorientation therein.

6. The method of making silicon steel sheet material comprising forming a melt containing iron, about l-4% silicon and sulfur wherein about 0.005 to 0.1% by weight of sulfur has been added to the melt, solidifying said melt, producing a silicon steel sheet of desired thickness therefrom by alternately rolling and annealing the same, and annealing the thus obtained steel sheet to remove impurities therefrom and to develop preferred crystal orientation therein.

7. The method of forming thin gauge silicon steel sheet material comprising forming a melt containing iron, about 14% silicon and sulfur wherein about 0.005 to 0.1% by weight of sulfur has been added to the melt, producing a metal sheet of about 14 to mils thickness from said melt, rolling the thus produced metal sheet to a thin sheet having a thickness of about 3 to 6 mils, and annealing the thus produced thin metal sheet to refine the same and to develop grain-orientation therein.

8. The method of forming thin gauge silicon steel sheet material comprising forming a melt containing iron, about 14% silicon and sulfur wherein about 0.005 to 0.1% by weight of sulfur has been added to the melt, producing a metal sheet of about 14 to 100 mils thickness from said melt, rolling the thus pro-duced metal sheet to a thin sheet having a thickness of less than 7 mils, and annealing the thus produced thin metal sheet to refine the same and to develop grain-orientation therein.

9. Silicon steel sheet material less than 7 mils thick as made by the method defined in claim 8.

References Cited in the file of this patent UNITED STATES PATENTS Littmann June 14, 1949 Maxwell Oct. 2, 1956 OTHER REFERENCES 

1. THE METHOD OF MAKING SILICON STEEL SHEET MATERIAL COMPRISING FORMING A MELT CONTAINING IRON, ABOUT 1-4% SLILCON AND SULFUR WHEREIN ABOUT 0.005 TO 0.1% BY WEIGHT OF SULFUR HAS BEEN ADDED TO THE MELT, PRODUCING A METAL SHEET FROM SAID MELT, AND ANNEALING SAID METAL SHEET TO REFINE THE SAME AND TO DEVELOP GRAIN-ORIENTATION THEREIN.
 8. THE METOD OF FORMING THIN GAUGE SILICON STEEL SHEET MATERIAL COMPRISING FORMING A MELT CONTAINING IRON, ABOUT 1-4% SILICON AND SULFUR WHEREIN ABOUT 0.005 TO 0.1% BY WEIGHT OF SULFUR HAS BEEN ADDED TO THE MELT, PRODUCING A METAL SHEET OF ABOUT 14 TO 100 MILS THICKNESS FORM SAID MELT, ROLLING THE THUS PRODUCED METAL SHEET TO A THIN SHEET HAVING A THICKNESS OF LESS THAN 7 MILS, AND ANNEALING THE THUS PRODUCED THIN METAL SHEET TO REFINE THE SAME AND TO DEVELOP GRAIN-ORIENTATION THEREIN. 